1. Introduction
Fractures and faults are common structural elements of shallowly deformed rocks. They result from processes controlled by physical forces and/or chemical potentials and by the rheology of rocks. These elements might form at all stages of rock history, from early/burial diagenesis to major tectonic events, and play a fundamental role in fluid migration and accumulation within the upper crust. Fractures and faults hence are the focus of detailed studies conducted during exploration and production/storage of subsurface renewable and fossil energy, as well as for waste repositories assessment. In fact, defining the parameters that control fracturing and faulting in rocks is key for a better understanding of fluid–rock interactions, time-dependent stress evolution and present-day earthquake hazards. The dynamic feedbacks between fluid flow, permeability rise/fall, chemical reactions and rock failure, however, are difficult to decipher, and remain challenging tasks to tackle. For example, the role of chemistry in fracture pattern development has recently been emphasized by Laubach et al. (Reference SE, RH, LJ, LM, JL, RM, JN, W, Evans, Kerisit, Olson, Dewers, Fisher, Bodnar, Evans, Dove, Bonnell, Marder and Pyrak-Nolte2019), who documented that a fracture may grow under subcritical stress conditions in response to the chemical weakening occurring at its tip, hence at a much lower stress level than would otherwise be required. Embrittlement enhanced by early diagenesis in carbonates has been shown to trigger shallow, early, low-stress, burial fractures in platform carbonates (LaBruna et al. Reference La Bruna, Lamarche, Agosta, Rustichelli, Giuffrida, Salardon and Marié2020). The current structural configurations must therefore be interpreted as the result of coupled mechanical and chemical processes, which took place in the past with changing environmental conditions (temperature T, pressure P, fluid pressure P f) over geological timescales (e.g. Ankit et al. Reference Ankit, Urai and Nestler2015). Ongoing work on the origin, occurrence, 3D spatial arrangement, evolution and mechanics of faults and fractures and their controlling parameters (e.g. Guiton et al. Reference Guiton, Sassi, Leroy and Gauthier2003; Lamarche et al. Reference Lamarche, Lavenu, Gauthier, Guglielmi and Jayet2012; Lavenu et al. Reference Lavenu, Lamarche, Gallois and Gauthier2013; Lavenu & Lamarche, Reference Lavenu and Lamarche2017; Laubach et al. Reference SE, RH, LJ, LM, JL, RM, JN, W, Evans, Kerisit, Olson, Dewers, Fisher, Bodnar, Evans, Dove, Bonnell, Marder and Pyrak-Nolte2019; Prabhakaran et al. Reference Prabhakaran, Bruna, Bertotti and Smeulders2019, Reference Prabhakaran, Bertotti, Urai and Smeulders2021; Passchier et al. Reference Passchier, Passchier, Weismüller and Urai2021; Correa et al. Reference Correa, Marret and Laubach2022) will provide new insights into the role these structures exert on the storage and migration paths of geofluids, natural hazards and overall shaping of the Earth’s surface. This knowledge is fundamental to mitigate the risks associated to exploitation of subsurface energy, as well as the risks associated to earthquakes and landslides, and to better preserve water resources hosted within fractured reservoirs. These themes deserve a close inspection by Earth scientists, who will take advantage of the opportunities provided by advancing technologies to better decipher the physical–chemical–mechanical processes occurring at depth in the upper crust.
The geometrical and structural properties of fractures and faults are very difficult to assess for underground rocks due to lack of resolution for detailed interpretations. Rocks with a high value of primary porosity commonly include deformation bands (Aydin, Reference Aydin1978), which might dramatically reduce the fluid storage and migration properties of the rock mass. Conversely, those characterized by a low amount of primary porosity often deform by means of combined opening-mode and/or pressure solution-assisted deformation mechanisms (Rispoli, Reference Rispoli1981). All resulting structural elements form sub-seismic-scale heterogeneities affecting the reservoir characteristics. However, the interplay among them often forms complex structural networks that are difficult to characterize in terms of resulting bulk petrophysical properties. In this regard, discrete fracture network (DFN) modelling of geocellular volumes representative of the subsurface rock volumes has been employed in the last two decades as a valid tool to compute the values of fracture porosity and of an equivalent permeability. Furthermore, the advent of 3D virtual outcrop models in conjunction with the proliferation of unmanned aerial vehicles has boosted the use of multiscalar 3D fracture data from large-scale outcrop (e.g. Bistacchi et al. Reference Bistacchi, Balsamo, Storti, Mozafari, Swennen, Solum, Tueckmantel and Taberner2015; Bisdom et al. Reference Bisdom, Nick and Bertotti2017; Corradetti et al. Reference Corradetti, Tavani, Parente, Iannace, Vinci, Pirmez, Torrienri, Giorgioni, Pignalosa and Mazzoli2018), and multiscalar fracture network attributes retrieved from the analysis of 2D satellite and aerial images (e.g. Palamakumbura et al. Reference Palamakumbura, Krabbendam, Whitbread and Arnhardt2020; Ceccato et al. Reference Ceccato, Tartaglia, Antonellini and Viola2022) are nowadays routinely used to generate DFN (e.g. Healy et al. Reference Healy, Rizzo, Cornwell, Farrell, Watkins, Timms, Gomez-Rivas and Smith2017).
Giuffrida et al. (Reference Giuffrida, Agosta, Rustichelli, Panza, La Bruna, Eriksson, Torrieri and Giorgioni2020) showed the problems arising in the computation of the scaling factors, as well as in the detailed characterization of both mechanical and hydraulic apertures of single fracture sets. Fault zones can be simplistically considered as comprising two main domains commonly labelled as fault cores and damage zones (Caine et al. Reference Caine, Evans and Forster1996; Faulkner et al, Reference Faulkner, Jackson, Lunn, Schlische, Shipton, Wibberley and Withjack2010). The fault cores include both main slip surfaces and breccias, cataclasites and gouges, and are often associated with syn-tectonic veining and mineralization. In the brittle regime, fault core rocks often form due to cataclasis, a process responsible for grain size reduction, grain shape evolution, and production of a powder-like matrix due to microscale intragranular extensional fracturing and chipping. Ferraro et al. (Reference Ferraro, Agosta, Ukar, Grieco, Cavalcante, Belviso and Prosser2019, Reference Ferraro, Agosta, Prasad, Vinciguerra, Violay and Giorgioni2020) showed that the diagenetic history of cataclastic fault rocks profoundly affects their present-day porosity and permeability properties, supporting the idea that structural diagenesis analysis (Laubach et al. Reference Laubach, Eichhubl, Hilgers and Lander2010) is required to fully assess the control exerted by localized strain on the storage and migration fluid properties of fractured reservoirs. The fault damage zones commonly flank the fault cores, and consist of fractured and fragmented rock volumes still preserving the original fabrics. Across the latter zones, even if many studies have documented common trends of both fracture density and intensity, fracture parameters do not always scale proportionally with fault displacement (Faulkner et al. Reference Faulkner, Mitchell, Healy and Heap2006; Balsamo et al. Reference Balsamo, Clemenzi, Storti, Solum and Taberner2019; Volatili et al. Reference Volatili, Agosta, Cardozo, Zambrano, Lecomte and Tondi2022) likely due to the heterogeneous stress distribution around the evolving main slip surfaces (Cowie & Scholz, Reference Cowie and Scholz1992; Mayolle et al. Reference Mayolle, Soliva, Dominguez, Wibberley and Caniven2021).
In the upper crust, fractures are the most common response of deformed rocks submitted to tectonic stresses (e.g, Hancock, Reference Hancock1985; Tavani et al. Reference Tavani, Storti, Lacombe, Corradetti, Muñoz and Mazzoli2015) and they have long been used as palaeostress indicators (Gauthier & Angelier, Reference Gauthier and Angelier1986). Interpretation of fractures and faults in terms of palaeostress analysis has received continuing attention over the last decade (Gudmundsson and Bergerat, Reference Gudmundsson and Bergerat2012; Lacombe, Reference Lacombe2012; Simon, Reference Simón2019; Pascal, Reference Pascal2021), with ongoing improvements of techniques to reconstruct the stress history from inversion of populations of veins and mesoscale striated faults (Shan et al. Reference Shan, Zheng, Hu and Yeh2021; Pascal et al. Reference Pascal, Jaques and Yamaji2022). A blind spot for most of these analyses has long been the lack of constraints on the absolute timing of brittle deformation. This absolute timing is never resolved through detailed inspection of the abutting/cross-cutting relations among individual fracture and fault sets, so the relevance and meaning of specific sets with respect to regional-scale deformation has been disputable, especially in regions that underwent polyphase tectonics, stress and/or block rotations and stress perturbations (e.g. Beaudoin et al. Reference Beaudoin, Lacombe, Roberts and Koehn2018). Recent improvements of absolute dating techniques have allowed for major progress in constraining the rate of development and lifetime of individual fracture and fault sets, as well as the timing of fluid flow in fractured rock volumes. Dating of shallow crustal faults may be achieved by geochronology of fault-generated materials such as fault gouge, slip-surface hematite, opal, pseudotachylyte and calcite steps/slickenfibres. A fault gouge forms by a combined process of mechanical comminution and in situ instantaneous neoformation of illite that is related to the energetics of faulting and fluid flow during faulting. Thus, isotopic dating of illites in fault gouge by K–Ar or Ar–Ar geochronology might provide a powerful tool to directly date the main episodes of fault slip depending on our technical ability to characterize the clay populations and to quantify the amounts of detrital (wall-rock inheritance) and authigenic phases (related to one or to multiple clay-generating slip events) (van der Pluijm et al. Reference van der Pluijm, Hall, Vrolijk, Pevear and Covey2001; Viola et al. Reference Viola, Scheiber, Fredin, Zwingmann, Margreth and Knies2016). Alternatively, vein opening and fault activity can be dated by U–Pb geochronology of syn-tectonic calcite cements (Roberts et al. Reference Roberts, Drost, Horstwood, Condon, Chew, Drake, Milodowski, Mclean, Smye, Walker, Haslam, Hodson, Imber, Beaudoin and Lee2020). However, fault rocks and veins may show a variety of calcite cements, and may also be affected by fluid–rock interactions subsequent to syn-tectonic cement precipitation, for instance during rock exhumation from depth (e.g. Aubert et al. Reference Aubert, Philippe, Lamarche and Salardon2020). For this reason, microstructurally constrained geochronology analyses are required to ensure that the sampled calcite cements grew syn-kinematically, and careful geochemical, compositional and microstructural characterization is necessary to guarantee that the dated isotopic system has remained undisturbed by later thermal or fluid-related activity (Roberts & Holdsworth, Reference Roberts and Holdsworth2022). Beyond providing absolute ages of fault slip and fracture development, geochronological dating of calcite cements from faults and veins has paved the way to estimate the duration of contractional stages associated with the folding event (Lacombe et al. Reference Lacombe, Beaudoin, Hoareau, Labeur, Pecheyran and Callot2021). Alternatively, coupling the Δ47CO2 temperature of cement precipitation in a closed fluid system to a 1D burial-time model yields the absolute timing of veins that formed during these contractional stages (Labeur et al. Reference Labeur, Beaudoin, Lacombe, Emmanuel, Petracchini, Daëron, Klimowicz and Callot2021).
Fluid sources, modalities of fluid flow and degree of fluid–rock interactions vary spatially and temporally as a function of crustal structural evolution. For this reason, the characterization of the fluid system (origin and nature of the fluid; T, P, timing of mineral precipitation, degree of fluid–rock interaction, hydraulic structure of the fractured reservoirs) and its evolution through time remains a challenging task (Aubert et al. Reference Aubert, Lamarche and Léonide2019, Reference Aubert, Lamarche and Leonide2021; Petit et al. Reference Petit, Chemenda, Minisini, Richard, Bergman and Gross2022). Fluids affect crustal rock strength by permitting pressure solution and chemical reactions to occur, thus stiffening the rock by precipitating cements and/or weakening it through development of low-strength hydrated mineral phases such as phyllosilicates. Fluids also reduce rock strength by overpressure. Conversely, the structural permeability architecture influences fluid migration pathways. The relations among fluid migration, fluid storage and rock deformation still remain to be further documented, quantified and integrated in comprehensive models of short-term and long-term fault and fracture behaviours and crustal mechanics, with implications for mineralization pattern and geothermal resources in ancient and active structurally complex, tectonic settings (e.g. Clemenzi et al. Reference Clemenzi, Storti, Balsamo, Molli, Ellam, Muchez and Swennen2015). The occurrence and time–space evolution of supra-hydrostatic fluid pressures are also key issues for understanding processes such as inherited fault reactivation, seismic swarming, gravitational sliding of rocks and pervasive hydrofracturing (Guglielmi et al. Reference Guglielmi, Nussbaum, Cappa, De Barros, Rutqvist and Birkholzer2021). Fluid (over)pressure is also a governing factor for the evolution of permeability and porosity of rocks and the generation, maturation and migration of economic fluids like hydrocarbons or ore forming hydrothermal fluids, and is therefore a key parameter in basin modelling. Performing and combining new techniques for better quantifying and modelling the role of fluid overpressures and their spatial/temporal variations in sedimentary basins (Berthelon et al. Reference Berthelon, Brüch, Colombo, Frey, Traby, Bouziat, Cacas-Stentz and Cornu2021), but more broadly in the crust in diverse environments, e.g. magma-driven emplacement of sills and dikes, is a challenge for the forthcoming years for both academy and industry.
The occurrence, patterns and mechanics of faults and fractures and their relation to chemistry, fluid flow and stress therefore are long-lasting, intriguing and fundamental topics in the geosciences owing to their importance for both academy and industry. The guest editors of this Special Issue have been involved in the production of several dedicated volumes on these topics during the last decade (Agosta & Tondi, Reference Agosta and Tondi2010; Smith et al. Reference Smith, Agosta, Toussaint and Holdsworth2012; Lacombe et al. Reference Lacombe, Tavani and Soto2012, Reference Lacombe, Swennen and Caracausi2014; Lacombe & Bergerat, Reference Lacombe and Bergerat2013; Iannace et al. Reference Iannace, Storti, Antonellini, Agosta and Tondi2015; Agosta et al. Reference Agosta, Luetkemeyer, Lamarche, Crider and Lacombe2016; Tondi et al. Reference Tondi, Iannace, Storti, Antonellini, Agosta and Mazzoli2016; Lacombe & Rolland, Reference Lacombe and Rolland2016; Laubach et al. Reference Laubach, Lamarche, Gauthier, Dunne and Sanderson2018; Agosta et al. Reference Agosta, Longhitano, Mazzoli, Minelli and Tondi2019; Balsamo et al. Reference Balsamo, Soliva, Eichhubl, Williams and Krawczyk2020). This new Special Issue follows up a successful session in the European Geosciences Union (EGU) Vienna 2021 meeting and gathers articles from scientists from diverse countries working in the field, in the lab and on simulations, and belonging to both industry and academy. The 28 articles, either original research or review, cover various fields and disciplines including structural geology, geomechanics, isotope geochemistry, hydrogeology and numerical modelling, with the common goal of better appraising and comprehending the development of fracture and fault systems in both space and time, as well as their coevolution and reciprocal interactions with underground fluids in a variety of geological settings. The papers provide enlightening examples of multidisciplinary research carried out by an international scientific community and offer a showcase for the efforts made by earth science researchers to assess the fault- and fracture-controlled fluid flow and stress state distribution in the subsurface. It is our hope that the present Special Issue will not only be of interest to the structural geologists, geophysicists and geochemists dealing with upper crustal rocks, but also be of use to the wider geoscience community.
2. Content of the volume
The volume is divided into the following five chapters:
Fracture occurrence, patterns and properties
In their paper, Bowness et al. study the lithostratigraphic and 3D fracture arrangement in layered rocks in a Cretaceous carbonate succession at Canyon Lake Gorge, Texas. Layers have variable clay and silicate minerals vs carbonate content. The authors show the sensitivity of fault and fracture arrangement to the mineralogy as well as the inhibition of fracture propagation when the thickness of incompetent layers increases. These results are of prime importance for modelling underground fractured reservoirs, because fracture connectivity, and therefore fluid flow, is shown to be dependent not only on bed thickness, but also on rock lithology and its mechanical-stratigraphic settings.
Manniello et al. use the case study of the Mesozoic shallow-water carbonates of Agri Valley (southern Italy) to assess the influence exerted by depositional and diagenetic heterogeneities on fracture geometry, distribution and multiscale properties. Stratigraphic, petrographic, mineralogical and mesoscale structural analyses indicate that burial-related, physical–chemical compaction and cementation processes have remarkably conditioned the evolution of the fracture stratigraphy of the studied carbonates.
Mercuri et al. analyse the intensity, distribution and orientations of the pre-folding fracture pattern affecting the Pietrasecca Anticline (Apennines, Italy). They recognize longitudinal and transverse joints, oriented approximately perpendicular to bedding, which predated the development of the pressure-solution cleavage and the intensity of which is not correlated with the structural position along the anticline. These observations suggest that jointing occurred in a foredeep environment before the Pietrasecca Anticline growth, which warned about the potentially erroneous interpretation of joints striking parallel and orthogonal to the main fold axis as representing syn-folding deformation structures.
Richard et al. use a case study of a carbonate reservoir in Egypt to illustrate the discrepancy between observations required to build a thorough geological understanding of the subsurface and the simplification imposed by modelling in fractured reservoirs. Observations on reservoir architecture, fracture typology and fracture connectivity are integrated with the regional geological context to build a conceptual fracture model, which is then drastically simplified to generate 3D discrete fracture network scenarios that are calibrated using exploration wells.
Formenti et al. investigate a natural outcrop located in Ontario, Canada, along the Niagara Escarpment. The outcrop exposes Palaeozoic sedimentary rocks cross-cut by high-angle fractures, which disarticulate large rock blocks and pose a serious hazard for the local population due to rock fall. The authors analyse and quantify the natural fracture network by means of combined field and numerical analyses. Specifically, they carry out discrete fracture network modelling of a geocellular volume representative of the study rock volume. Results show that the high degree of fracture connectivity contributes to the local geohazard. In fact, together with bedding planes that might act as free surfaces, the well-connected fractures bound cuboid rock blocks prone to rock fall in the presence of circulating meteoric water.
Fracture mechanics
Bons et al. review the formation and evolution of non-igneous hydrofractures. The authors show that both Terzaghi’s and Biot’s theories can be reconciled if the appropriate boundary conditions are considered. They discuss the propagation of hydrofractures after initial rock failure, and address the question of how to ascertain whether a fracture is a hydrofracture or not. As a result, the authors propose that extensional or dilational fractures forming under normal circumstances below 2–3 km depth are often hydrofractures, whereas at shallower depths usually they are not. Furthermore, they critically assess which vein structures in the geological record can, and which do not necessarily, indicate hydrofracturing.
Gudmundsson investigates how hydrofractures select their propagation paths, particularly in layered and faulted rocks, and proposes that among all the possible paths, a hydrofracture selects the path of minimum action as determined by Hamilton’s principle. This means that the selected path is that along which the energy released multiplied by the time taken for the propagation is a minimum. Hydrofractures advance their tips/fronts in steps, with a time lag between the fracture front and the fluid front, each step being controlled by Hamilton’s principle. Hydrofracture path is expected to be everywhere perpendicular to the trajectory of σ3 and along the trajectory of σ1 in homogeneous rocks, faults may be used as part of this path primarily if the fault is steeply dipping and with close to zero tensile strength.
Hobbs and Ord address the implications of adding a cap to the yield surface for elastic–plastic and elastic–viscoplastic solids with coupling between deformation, fluid flow and mineral reactions. A capped yield surface means that a deforming fluid-saturated rock mass automatically oscillates between a low stress – high fluid pressure, low-permeability state and a high stress – low fluid pressure, high-permeability state controlled by competition between dissolution and precipitation. Failure at the cap results in opening-mode veins normal to compression in low-permeability rocks if the deformation is coupled to mineral reactions with negative ΔV in the rock matrix. This mode-switching process is a coupled mineral reaction – deformation – fluid flow cyclic mechanism that is an alternative to the fault-valve process. It does not depend on fluids breaching a permeability ‘seal’ and is intrinsically aseismic, although it can, in principle, nucleate a seismic cycle process.
Hooker et al. show that calcite veins from the Marcellus Formation (USA) grew via a combination of continuous fibrous growth related to pressure-solution creep and growth of a pressure fringe around a pre-existing, sealed fracture, and punctuated fracture-opening increments linked to an overpressured, mineral-saturated fluid. Calcite stable isotopic signatures support a regional opening-mode vein set that formed in response to catagenetic fluid overpressures within a tectonically imposed regional stress field.
Massaro et al. propose a new granular rock for simulation of multiscale fault and fracture processes in analogue experiments. They used silica sand and hemihydrate powder to form cohesive aggregates capable of deforming by tensile and shear failure under variable stress conditions, providing suitable conditions to study fracture processes in fault zones during different stages of fault evolution in dynamically scaled analogue experiments.
The paper by Gage et al. deals with the winter weathering of fractured sedimentary rocks on the Niagara Escarpment, Ontario. They examined the temperature of the rock surface and fractures at three in situ sites for >1 year, concluding that site-specific factors and pre-existing fractures moderate the influence of air temperature and insolation on thermal gradients, ultimately controlling the overall weathering regime.
Absolute dating of faults and fractures
Parizot et al. address the question of whether intraplate deformation related to far-field stress transmission from distant plate boundaries occurs during tectonic pulses marked by high strain rates at plate boundaries, or if it corresponds to low-intensity but regional continuous deformation through time. To do so, the authors have dated syn-kinematic calcite cements using U–Pb geochronology along the Cévennes Fault System (SE France). Following an Albian activity, the fault system underwent a continuous compressional activity during the whole Eocene and probably during the Late Cretaceous – Palaeocene, which emphasizes that intraplate deformation occurred rather continuously and not only during pulses of high rates of contraction at the Iberian–Eurasian plate boundary as previously interpreted from the sedimentary record.
Blaise et al. constrain the timing of brittle deformation and associated calcite cementation in Jurassic carbonates of the eastern Paris basin by means of U–Pb geochronology. They show that almost all the sampled veins were nearly continuously cemented between 48 and 33 Ma by low-temperature meteoric fluids. Most veins oriented NNE–SSW formed either in response to the far-field transmission of Pyrenean compressional stress (48–43 Ma) or in response to extension related to European Cenozoic Rift System (ECRIS) (∼35–33 Ma), with some veins recording the progressive transition from compression to extension (43–35 Ma). A few late veins oriented NNW–SSE and dated 32 and 18 Ma may be related to either the Alpine compression or to late Pyrenean deformation.
Torgersen et al. reconstruct multiple brittle reactivations of a long-lived, regional-scale mylonitic shear zone in SE Norway by integrating structural and micro-textural analyses with K–Ar and 40Ar–39Ar geochronology. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core provided a combination of careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis is conducted. Further, brittle faulting is controlled by mechanical anisotropy of parallel-oriented, through-going phyllosilicate-rich foliation planes within the mylonitic fabric.
Pavlovskaia et al. reconstruct the tectonic evolution of the northern Verkhoyansk fold-and-thrust belt in the Siberia Craton using palaeostress data and U–Pb calcite dating on fault slickenfibres. They documented changes in the stress regimes encompassing Early Permian strike-slip faulting, Early Cretaceous compression, Late Cretaceous compressional reactivation and finally late-stage Palaeocene extension.
Fluid flow and mineralization in faults and fractures
Beaudoin et al. provide a critical review of the geochemical techniques applied on syn-kinematic calcite cements and associated trapped fluids and show how they can constrain the temperature, pressure, origin and pathways of fluids during deformation and allow the characterization of the past fluid system. Taking orogenic forelands as a playground, they show that the past fluid system generally evolves from being rather closed to external fluids when deformation is bounded to mesoscale structure development to opening to vertical flow when thrusts and folds develop. The past fluid system also evolves under the influence of the structural style, fault geometry and lithology of the sedimentary succession. This review illustrates the concept of geochemistry-assisted structural geology, with the geochemistry of calcite cements constraining subsurface geometries and structural development.
Dielforder et al. analysed the 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units in the Swiss Alps and show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade, which reflects the progressive equilibration of the pore fluid with the host rock. This paves the way to the reconstruction of the prograde to early retrograde tectonic evolution of a fold-and-thrust belt and helps understand the relative timing of deformation events.
Gottardi and Hughes address the role of fluids in mid-crustal shear zones in the Raft River Mountains, Utah, USA, documenting ductile overprint of brittle microstructures. Such results suggest that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.
The paper by Berger and Herwegh deals with deformation and fluid flow processes in the Grimselpass Breccia Fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). They documented feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, thus presenting a case study for the long-term preservation of orogenic hydrothermal system.
Zucchi et al. describe in detail a fossil exhumed geothermal system exposed in eastern Elba Island, that can be an analogue for geothermal systems in a polyphase folded and faulted setting and refines the geothermal conceptual models. The fault zones and the extensional jogs at their tips channel the deep fluids. Pre-existing normal and oblique-slip faults served as feeder conduits for geothermal fluids and hydraulically connected the metasiliciclastic rock bodies previously deformed by two generations of folds, leading to reprecipitation of quartz, Fe-oxides and sulphides.
Smith et al. focus on the digital structural analysis of Late Jurassic rocks of the Franconian Basin (Germany). They make use of an open pit quarry to investigate both geometry and distribution of the fracture network cropping out at different sites located at various distances from a large-scale reverse fault. Results are employed in a finite-element modelling aimed at simulating the fluid flow through the fracture medium. The authors assess the control exerted by the fault-related fractures on the size and shape of the permeability ellipses derived from the modelling. The permeability ellipse computed for the outcrop close to the reverse fault is elongated parallel to the main slip surface, whereas those computed for the outcrops away from the fault do not show significant anisotropy.
By means of geochemical modelling, Koehn et al. explore the modalities of outcrop-scale ore mineralization due to flow of a metal-rich fluid through a faulted rock volume. The authors simulate a number of scenarios, which deal with the fluid infiltration through a low-permeability rock cross-cut by two high-permeability fault zones. By solving the advection–diffusion equation for 12 chemical species at varying temperatures, the authors document that ore mineralization localizes within the fault zones, specifically at the boundary with the encompassing host rock. Results are therefore consistent with mineral precipitation depending on the dominant transport process, with mineralization predominant where local diffusion dominates.
Faults, fractures and stress
Tranos and Osman describe previously ignored or falsely interpreted hydroplastic structures, which are ‘odd’ kinematic indicators in the basal part of the Eocene Middle Rus Formation. These structures occur in relation to a principal displacement zone along the boundary/interface between the Lower and Middle Rus, the so-called Rus soft-sediment detachment, that displays transport/slip towards the NNW. Palaeostress analysis indicates an Andersonian transtensional stress regime, under which the activation of the Rus soft-sediment detachment would hardly occur except for a negative effective principal stress σ3 and an abnormally low frictional coefficient caused by fluid pressure. The soft-sediment Rus detachment is interpreted as an early (Late Eocene) witness of the inception of the Zagros collision.
Assie et al. investigate the late Cenozoic evolution and deformation of the Fanshi Basin, (Shanxi rift, North China Craton). By means of palaeostress analysis of mesoscale fault-slip data, they unravel a multiphase deformation history since late Cenozoic time, including possible σ2/ σ3 stress permutations related to structural inheritance: (1) a Neogene strike-slip stress (σ1 NE–SW and σ3 NW–SE), activating the large NNE faults as right-lateral strike-slip faults; (2) a short-lived early Pleistocene NE–SW extension; and (3) a NW–SE to NNW–SSE extension continuing since the late Pleistocene and giving the current half-graben geometry of the Fanshi Basin by activating the Fanshi fault as a normal fault in the southern part of the graben.
Köhler et al. reconstruct the spatio-temporal variations of post-Triassic palaeostress fields in the Franconian Platform (SE Germany) using stylolites and mesoscale faults. Two cycles of stress evolution, from normal stress to thrusting, strike-slip and back to normal, are reconstructed, recording the stress evolution in an intraplate compressional setting during Mesozoic Africa–Iberia–Europe convergence and later Cenozoic Alpine orogeny.
Bhowmick and Mondal present a field study of veins, fractures and faults from metavolcanic rocks of the Chitradurga region (India). Dilation and slip tendency, and fracture susceptibility analysis allows the authors to recognize that pre-existing anisotropies represented the main discontinuities for fracture propagation and vein emplacement, which has occurred during multiple cycles of fluid pressure variation.
Zhao et al. investigate the present-day stress field in a small rock volume of the Tarim Basin, China, cross-cut by a sub-vertical, strike-slip fault zone. The authors build a 3D geomechanical model aimed at assessing the possible control exerted by the sub-vertical fault zone on the stress state. They show that the direction of the maximum horizontal principal stress axis is deflected near the fault zone, aligning almost parallel to the fault strike. Results are therefore discussed to emphasize the role exerted by local stress fields on the propagation direction of hydraulic fractures, an attitude in the subsurface that is not easy to predict in the vicinity of large fault zones.
Maerten et al. describe and apply a new multiparametric inversion technique based on geomechanics that can invert for both the far-field stress attributes and the internal pressure of magma chambers or stocks, constrained by observed dike or eruptive fissure orientations. The technique uses linear elastic models that are solved using a 3D boundary element method. Then the effectiveness of this technique and its practical use is demonstrated through its application to natural examples.
Acknowledgements
The Guest Editors would like to thank all the reviewers of the manuscripts submitted to this Special Issue, who have played a crucial role in maintaining a high level of rigour to the scientific contributions. Miss Susie Cox is warmly acknowledged for the great help she provided during the whole editorial process.
